1. Field of the Invention
The present invention relates to an FSK (Frequency Shift Keying) demodulator for demodulating an FSK signal.
2. Description of the Related Background Art
A typical frequency shift keying demodulator (hereinafter referred to as an FSK demodulator), which is used in wireless communication receivers, receives an input FSK signal to acquire, as data, a detected signal of an amplitude associated with the received frequency. The FSK demodulator then compares the amplitude of the detected signal with a threshold value to make an evaluation of the data. A data error may occur at the FSK demodulator due to the effect of noise produced when the detected signal is acquired or due to an offset caused by a frequency error at a local oscillator that is included in a transmitter or receiver.
To suppress the noise effect produced when a detected signal is acquired, a method is available for performing demodulations without using a signal around the zero cross point that is susceptible to noise (e.g., see Japanese Patent Application Kokai No. Hei 6-54004.)
In some methods typically available for removing offsets, a detected signal is introduced into an integrator to acquire the average of amplitude values of the detected signal, which is in turn determined as the quantity of offset.
Another method for removing offsets is also disclosed, for example, in Japanese Patent Application Kokai No. Hei 7-231338 in which a sign evaluation point is set past the center of a symbol period. Still another method for removing offsets is disclosed, for example, in Japanese Patent Application Kokai No. 2001-358786 in which upon acquirement of a detected signal, a cycle is determined on the basis of the intersection between the time axis and the sampled value of an FSK signal waveform to compute the frequency from the cycle. A further method for removing offsets is also disclosed in Japanese Patent Application kokai No. Hei 11-298541 in which a sample value of the detected signal at the zero cross point is subtracted from the detected signal.
According to the method for determining the average value of a detected signal as the quantity of offset, a system in which “0” and “1” equally appear in data provides a stable average value between “0” and “1.” However, in a system in which “0” and “1” do not equally appear, the output from the integrator, i.e., the quantity of offset may vary depending on the data pattern, thereby causing the offset not to be properly removed.